Conference Proceedings Paper
SORPTION AND SPECIATION OF PALLADIUM UNDER HIGH IONIC STRENGTH CONDITIONS
3rd Canadian Nuclear Waste Management Decommissioning and Environmental Restoration - 2016 Sept. 11-14
J. Riddoch (McMaster University)
J. Goguen (McMaster University)
S. Nagasaki (McMaster University)
Sedimentary rocks are being considered as potential host rocks for a deep geological repository in Canada. Deep-seated sedimentary rocks in Canada have been observed to contain highly saline ground and pore waters, the relatively high ionic strength (I) of which may influence speciation and rock matrix sorption properties. To this end laboratory sorption experiments have recently conducted palladium (Pd) on sodium bentonite, illite and Ordovician age shale as a function of pH and solution ionic strength. pH values in the range of 5 to 9 were considered and the I ranged from 0.1 to 4 M. The experiments were carried out under aerobic conditions at 25 ̊C, and the Eh value of the solution ranged from 470 to 500 mV. The data from sorption experiments were used to validate surface complexation models developed in PHREEQC with the JAEA thermodynamic database (TDB). The sorption of bentonite, shale and illite all showed strong dependence on I. Sorption of all solids decreased with increasing I, however the rate of decrease was greatly reduced beyond an I of 2 M. The surface complexation models for montmorillonite (the major constituent clay mineral of sodium bentonite), and illite (the major constituent clay mineral of shale) also showed strong dependence on I. The sorption of bentonite, shale and illite also showed dependence on pH. At I of 0.1M and 1M sorption decreased with increasing pH. At I of 4 M, sorption appears to peak around a pH of 7. The speciation of Pd was also evaluated by PHREEQC with the JAEA TDB. At low I (0.1 M), when pH is less than 6, negatively charged PdCl2- is the dominant species and when pH is greater than 6 neutral Pd(OH)2 is dominant. As I increases, the pH value of the transition point from PdCl2- dominance to Pd(OH)2 dominance increases.
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